As regards the production of underwater acoustic transducers, the use of optical fibers is a known solution which exhibits recognized advantages which include firstly the small bulk of a hydrophone produced using such technology, as well as the possibility of producing an assemblage of hydrophones on the basis of one and the same fiber by multiplexing on this same fiber the pressure variation information detected by the various hydrophones forming this assemblage, each hydrophone being associated with a given wavelength.
However, in order to obtain sensitivity sufficient to be capable of listening to a very low level of acoustic signal, typically a level below sea state noise 0, on the Knudsen scale, it is known that it is necessary to amplify the deformation of the optical fiber induced by the pressure wave by means of an appropriate acousto-mechanical device.
There currently exist two main classes of optical fiber hydrophones capable of listening to a noise level below sea state noise 0 and therefore potentially usable in hydrophone arrays or in antennas using hydrophones for underwater applications: optical fiber coil interferometric hydrophones and optical fiber laser cavity hydrophones.
In optical fiber coil interferometric hydrophones, the optical quantity measured is the optical phase variation aggregated over the total length of an optical fiber (20 to 100 m typically), coiled on a compliant mandrel configured in such a way that its diameter varies under the effect of the acoustic pressure to be measured, thereby inducing a dynamic variation in the length of the optical fiber wound thereon, an air mandrel for example.
The dynamic variation in the length of the optical fiber is manifested by a variation in the phase of the signal transported by the fiber. Thus, for a standard single-mode silica optical fiber, it may be shown that the relative variation of the phase of the signal is equal to about 0.78 times the relative deformation of the length of the fiber. This phase variation may be measured with the necessary precision by placing the hydrophone in the arm of an unbalanced optical fiber interferometer of Michelson type for example.
This type of hydrophone exhibits the main drawback of not being able to be sufficiently miniaturized. Indeed, in order to transmit the information relating to the pressure measurements carried out by each hydrophone constituting an array, various interrogation and multiplexing techniques well known to the person skilled in the art may be implemented. Their object is to make it possible to link, with the aid of a single optical fiber, all the associated hydrophones within one and the same array to the system charged with utilizing these measurements. However, the multiplexing on a single optical fiber of a large number of hydrophones requires that their insertion losses along the optical fiber be low, typically less than 1 dB. This implies that, in each hydrophone, the optical fiber is coiled on a mandrel exhibiting a minimum diameter, so as to limit to the minimum the losses related to the curvature of the fiber. Now, even by using microstructured optical fibers with a very low admissible radius of curvature (Fiber for FTTH applications), good performance in terms of sensitivity and insertion loss are difficult to achieve with a winding diameter of substantially less than 15 or 20 mm.
Moreover, insofar as each hydrophone constituting the array measures a simple pressure value, it is necessary to introduce into the hydrophone array, one or more optical fiber coil reference sensors, not subjected to the acoustic pressure, so as to circumvent by subtraction the variations in the intrinsic sensitivity of the optical fiber coil to static pressure and to temperature. This procedure exhibits the drawback of having to add further sensors and of limiting the domain of use of the hydrophone in immersion (limited dynamic range of the sensor).
Optical fiber laser cavity hydrophones make it possible to produce hydrophones of high sensitivity, that is to say capable of listening to low-level sea noise. Their implementation makes it possible to employ techniques of coherent optical processing to precisely measure the frequency variations of the optical signal conveyed by the fiber. Such a variation is here consequent upon the reception of an acoustic signal by a Bragg grating inscribed in an active optical fiber, an Erbium or Erbium/Yttrium doped optical fiber for emission around 1.5 μm for example.
In a known manner a Bragg grating includes, in proximity to its center, a phase jump of substantially equal to Tr so as to constitute a monofrequency laser cavity, the optical pumping of the cavity being achieved by way of a diode (pumping diode) which may be sited remotely a long distance away via a standard optical fiber. The emission frequency of the laser cavity thus constituted depends on the spacing of the Bragg grating and the central phase of the phase shift. Therefore, if the laser cavity is uniformly deformed, the emission frequency of the laser will vary in the same proportions with a coefficient of 0.78 related to the elasto-optical properties of the silica constituting the fiber.
It is thus possible to wavelength multiplex several hydrophones having laser cavities exhibiting different operating wavelengths, lying in the amplifying band of the doping material of the active optical fiber: about 40 hydrophones, for example, in the case of an Erbium doped fiber, for wavelengths lying on the 100 GHz ITU grid of band C of Erbium.
However, the axial deformation induced by an acoustic pressure applied directly to the external surface of the laser cavity is not sufficient to obtain hydrophone sensitivity compatible with a level of less than sea state noise 0, having regard to the PSD (power spectral density) of the intrinsic noise of the laser (of the order of 20 Hz/√{square root over (Hz)} to 1 kHz) which limits the noise of the interrogation system. It is indeed possible to show that it is necessary to have a hydrophone sensitivity of greater than at least 100 or 110 dB Hz/Pa in order to be capable of listening to a sea state noise 0 on the Knudsen scale in a band of about 10 Hz to 10 kHz.
Therefore, it is necessary to amplify the deformation of the cavity induced by the acoustic pressure by means of an acousto-mechanical device. Having regard to the intrinsic sensitivity of the laser cavity to a deformation the objective to be achieved for the acousto-mechanical device is of the order of a nanostrain/Pa with a stable response in a broad frequency band. Several known configurations make it possible to achieve these values. Nonetheless, in all these configurations the hydrophone retains an intrinsic sensitivity to temperature at least equal to that of the laser cavity together with sensitivity to static pressure, without being associated with a voluminous hydrostatic filter.
Thus, in the current state of the prior art, no technical solution belonging to one or the other of these two classes makes it possible to ensure, in an intrinsic manner, the insensitivity of an optical fiber hydrophone to variations in static pressure and to variations in temperature. Now, the operation of a hydrophone under varied environmental conditions (variation of immersion and of temperature, acceleration), is actually possible only if its behavior is actually independent of immersion depth, stated otherwise of static pressure, of temperature and of accelerations.
Moreover, in the case of arrays of sensors it is also very important to have acoustic sensors exhibiting stable interrogation wavelengths, so as to be able to access the various sensors via a single optical fiber by multiplexing and to carry out the wavelength multiplexing and demultiplexing of the information regarding pressure transmitted by the various sensors by simply using optical fiber passive components.